Compressively Stressed Medium Phosphorus Electroless Nickel

ABSTRACT

A method of producing compressive stressed, medium phosphorus nickel deposits from an electroless nickel plating solution is disclosed herein. It was previously believed that a compressively stressed nickel deposit could only be produced with nickel deposits having a high phosphorus nickel content. The inventors have surprisingly discovered that, through selection and concentration of the additives of an electroless nickel plating solution, a medium phosphorus nickel deposit may be produced exhibiting compressive stress.

FIELD OF THE INVENTION

The present invention relates generally to a medium-phosphorus electroless nickel plating solution and a method of using the same to produce a compressively stressed nickel deposit on a substrate.

BACKGROUND OF THE INVENTION

Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions plated on a base substrate. In electroless plating, use is made of a chemical reducing agent, thus avoiding the need to employ an electrical current as is required in electrolytic plating operations.

Deposits made by electroless plating have unique metallurgical characteristics. For example, the coatings may exhibit good uniformity, excellent corrosion resistance, wear and abrasion resistance, nonmagnetic and magnetic properties, solderability, high hardness, excellent adhesion, and low coefficient of friction. The deposits can be made on a wide range of substrates, including metallic surfaces such as steel, brass, aluminum, aluminum alloy, copper, titanium, titanium alloy, iron, magnesium, magnesium alloy, nickel, nickel alloy, bronze, or stainless steel, among others, and non-metallic surfaces such as plastics, including polyacrylates, polyimides, nylon, polyamides, polyethylene, and polypropylene, among others. In addition, because electroless plating deposits are autocatalytic, it is possible to uniformly plate substrates having complex shapes.

Electroless plating solution compositions typically comprise an aqueous solution containing metal ions to be deposited, catalysts, one or more reducing agents, one or more complexing agents, bath stabilizers and other plating additives, all of which are tailored to a specific metal ion concentration, temperature and pH range.

One of the most common electroless plating systems involves the electroless deposition of a nickel or nickel alloy onto a substrate. Plating solutions of this type typically comprise a source of nickel ions and a reducing agent, usually hypophosphite. The plating solutions may also include one or more complexing agents, buffers, brighteners when desirable, and various stabilizers to regulate the speed of metal deposition and avoid decomposition of the solution.

Nickel deposited from an electroless nickel plating solutions using hypophosphite as the reducing agent will contain elemental phosphorus alloyed with the plated nickel. The phosphorus content of the plated nickel deposit increases its corrosion resistance, adhesion, and shearing properties. However, the brightness and cosmetic appearance of the plated nickel deposit will decrease at high phosphorus contents. Therefore, the art typically distinguishes between so-called “medium phosphorus” and “high phosphorus” electroless nickel plating solutions. Medium phosphorus solutions retain some of the corrosion resistance, adhesion, and shearing properties of high phosphorus solutions, while also producing a brighter, more aesthetically pleasing deposit. For purposes of the present invention, a “medium phosphorus” electroless nickel plating solution is one that will produce a nickel deposit having between 4% and 9% phosphorus.

It is understood that plated metal deposits can contain “stresses” after plating is finished. These stresses can be either “tensile” or “compressive.” A tensile stress exists when the plated deposit is seeking to contract relative to the substrate. One can conceptualize this as the deposit being stretched on the substrate. Conversely, a compressive stress exists when the plated deposit is seeking to expand relative to the substrate. One can conceptualize this as the deposit being compressed like a spring on the substrate. Both tensile stress and compressive stress are measured in units of pounds per square inch (psi).

As a general matter, medium and low phosphorus electroless nickel plating solutions produce nickel deposits with a tensile stress. However, tensile stress has a propensity to decrease the deposit's corrosion and wear resistance. Over time, the tensile stresses can lead to “cracks” in the deposit. Therefore, a compressive stress is preferred to increase a nickel deposit's corrosion resistance.

It was generally understood in the art that in order to produce an electroless nickel deposit having a compressive stress one would need to produce a “high phosphorus” nickel deposit. Thus, for example, EP 0071436A1 discusses various techniques to increase phosphorus content to produce a compressively-stressed, high-phosphorus nickel deposit. In addition to improving corrosion and wear resistance, compressive stress also leads to enhanced adhesion of the nickel deposit and enhances the machining and meting properties of the nickel deposit. The art believed that it was not possible to produce a compressively-stressed medium-phosphorus nickel deposit.

SUMMARY OF THE INVENTION

The present inventors have surprisingly discovered a method for producing a compressively-stressed medium-phosphorus nickel deposit, thereby proceeding against conventional wisdom in the art.

Specifically, in one embodiment, the inventors have discovered a method of producing a compressively-stressed medium-phosphorus electroless nickel deposit on a substrate, the method comprising the step of contacting the substrate with an electroless nickel plating solution, the electroless nickel plating solution comprising:

i) nickel ions;

ii) hypophosphite ions;

iii) at least one chelator; and

iv) a molecule comprising divalent sulfur;

wherein the electroless nickel plating solution deposits on the substrate a compressively-stressed layer of nickel comprising between 4% and 9% phosphorus.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Nickel plating solutions made in accordance with a preferred embodiment of the present invention comprise nickel ions, hypophosphite ions, at least one chelator, and a molecule comprising divalent sulfur. The nickel plating solution may preferably comprise other additives, such as acids, buffering agents, metal ions other than nickel ions, accelerators, pH adjusters, stabilizers, and brighteners.

Nickel ions are preferably dissolved into the nickel plating solutions using one or more nickel-containing salts, and preferably nickel salts wherein nickel is prevent in its divalent oxidation state. Preferably, nickel chloride, nickel sulfate, nickel carbonate, and/or nickel acetate are used. The nickel content of the nickel plating solution may be set in a range from 1 gram per liter to 10 grams per liter. The inventors have discovered that using a nickel ion concentration at the lower end of this range is one factor that aids in the production of a nickel deposit having compressive stress. Therefore, the nickel content of the nickel plating solution is preferably set in a range from 1 gram per liter to 5 grams per liter, and more preferably in a range from 2 grams per liter to 4 grams per liter.

Hypophosphite is preferably used in the nickel plating solution as a reducing agent and as a source of phosphorus in the plated nickel deposit. Hypophosphite ions are preferably dissolved into the nickel plating solution using one or more hypophosphite containing salts. Sodium hypophosphite and potassium hypophosphite are particularly preferred. Sodium hypophosphite may be present in the nickel plating solution are a concentration in a range from 5 grams per liter to 50 grams per liter. The inventors have discovered that using a hypophosphite concentration at the lower end of this range is one factor that aids in the production of a medium phosphorus nickel deposit having compressive stress. Therefore, sodium hypophosphite is preferably present in the electroless nickel plating solution at a concentration in a range from 15 grams per liter to 25 grams per liter.

The nickel plating solution also preferably comprises at least one chelator, preferably selected from the group of monocarboxylic acids, dicarboxylic acids, hydroxycarboxylic acids, ammonia and alkanolamines. Chelators are preferably present in the solution at a concentration in a range of about 10 to about 80 grams per liter, more preferably in a range of about 20 to about 30 grams per liter. Chelators complex nickel ions and thus prevent excessively high concentrations of free nickel ions. As a result the solution is stabilized and the precipitation of for example nickel phosphite is suppressed. Chelators can also act as pH buffers in the nickel plating solution. Particularly preferred chelators include lactic acid, malic acid, glycine, acetic acid, and combinations of the foregoing. The inventors have discovered that choice, lower total concentration, and combination of chelators are factors that aid in the production of a medium phosphorus nickel deposit having compressive stress.

The nickel plating solution may also include one or more accelerators, such as glycine, fluorides, borides or anions of mono- and dicarboxylic acids. If used, the accelerator is present in the bath at a concentration in a range from 0.001 to 10 grams per liter. Accelerators can activate hypophosphite ions and thus accelerate deposition.

The nickel plating solution may also contain one or more stabilizers. Preferred stabilizers include lead, tin, arsenic, molybdenum, cadmium, bismuth, and/or thallium ions and/or thiourea. Stabilizers are used to prevent decomposition of the solution, by masking catalytically active reaction nuclei. If used, the stabilizer is used in the bath at a concentration in a range from 0.01 to 250 milligrams per liter.

The nickel plating solution may also preferably comprise one or more pH buffers, which may be a sodium salt of a chelator and/or also the associated corresponding acid to keep the pH constant for longer operating times. The buffer may also be a non-chelator additive to the nickel plating solution. The buffer is preferably present in the bath at a concentration in a range from 0.5 to 30 g/L.

The nickel plating solution may also comprise one or more pH regulators. Preferred pH regulators include acetic acid, sulfuric acid, hydrochloric acid, sodium hydroxide, sodium carbonate and/or ammonia. These may be added to the solution upon creation or may be added to the solution over time to regulate its pH. The pH of the nickel plating solution is preferably maintained within a range between 4 and 6, more preferably between 4.5 and 5.5.

The nickel plating solution may also comprise one or more sources of divalent-sulfur-containing molecules. Preferred divalent-sulfur-containing molecules include 2-Aminothiozole, thiourea, thiocyanate, and thiosalicylic Acid. The divalent-sulfur-containing molecules are preferably present in the nickel plating solution at a concentration ranging from 0.5 milligrams per liter to 10 milligrams per liter, and more preferably from 1 milligram per liter to 5 milligrams per liter. The inventors have discovered that choice, concentration, and combination of divalent-sulfur-containing molecules are factors that aid in the production of a medium phosphorus nickel deposit having compressive stress.

The nickel plating solution is preferably maintained at a temperature above room temperature during the process of plating. Particularly preferred temperatures are in the range of 175° F. to 200° F. The solvent used to create the electroless nickel plating solutions is preferable water.

The present invention is also related to a method of producing a compressively-stressed medium-phosphorus electroless nickel deposit on a substrate. Preferred substrates include metallic surfaces such as steel, brass, aluminum, aluminum alloy, copper, titanium, titanium alloy, iron, magnesium, magnesium alloy, nickel, nickel alloy, bronze, or stainless steel, among others, and non-metallic surfaces such as plastics, including polyacrylates, ABS, polyimides, nylon, polyamides, polyethylene, and polypropylene, among others. A particularly preferred substrate for purposes of the present invention is a steel surface. The substrate can be formed into any shape or size.

The method of the present invention comprises the step of contacting the substrate with an electroless nickel plating solution, preferably those previously described. Contact can be made in a variety of manners, such as by immersion or spraying. The solutions may also be agitated or stirred during immersion in manners known to persons of ordinary skill in the art. The substrate is preferably contacted by the electroless nickel plating solution for a time in the range of 10 minutes to 4 hours. For a strike layer of electroless nickel, the substrate is preferably contacted by the electroless nickel plating solution for a time in the range of 2 minutes to 10 minutes. The method of the present invention produces a nickel deposit exhibiting compressive stress, preferably a compressive stress between 200 psi and 5000 psi.

The following example is given to illustrate a preferred embodiment of the invention and to further describe and show potential advantages and unexpected results of practicing preferred embodiments of this invention. Nothing in the following example should be construed as a limitation on the invention as claimed below.

COMPARATIVE EXAMPLE

A solution of NiKlad ELV 847, a medium-phosphorus electroless nickel product available from MacDermid Enthone, Inc., was used as the electroless nickel plating solution. The solution was heated to a temperature of 190° F. and the operating pH was 5. A steel substrate was immersed into the electroless nickel plating solution for 60 minutes. A bright layer of nickel was deposited onto the substrate surface. The nickel deposit was analyzed, and it was determined to contain 6.4% phosphorus and to exhibit tensile stress of 4440 psi (pounds per square inch).

Example 1

An aqueous electroless nickel plating solution was created with the following composition:

Component Concentration Nickel ion 3 g/L Sodium Hypophosphite 20 g/L Lactic Acid 5.2 g/L Malic Acid 7.85 g/L Glycine 5.5 g/L Glacial Acetic Acid 9.15 g/L 2-Aminothiozole 2.5 mg/L Thiosalicylic Acid 1 mg/L Bismuth ion 2.5 mg/L

The solution was heated to a temperature of 190° F. and the operating pH was 5. A steel substrate was immersed into the electroless nickel plating solution for 60 minutes. A bright layer of nickel was deposited onto the substrate surface. The nickel deposit was analyzed, and it was determined to contain 6.5% phosphorus and to exhibit compressive stress of 2962 psi (pounds per square inch). This result was unexpected in view of the 4440 psi tensile stress produced in the Comparative Example. Not only was the type of stress changed from tensile to compressive, but the magnitude of the resulting compressive stress was found to be substantial (2962 psi). As discussed, it was not previously believed that a medium phosphorus electroless nickel deposit could exhibit compressive stress.

Typical and preferred embodiments have been described herein for purposes of illustration. The foregoing preferred embodiments should not be considered to limit the scope of the invention, which is described in the following claims as understood by one having ordinary skill in the art. Various alternatives, modifications, adaptations, and additions will occur to one skilled in the art without departing from the scope of the invention described by the claims herein. 

What is claimed is:
 1. A method of producing a compressively-stressed medium-phosphorus electroless nickel deposit on a substrate, the method comprising the step of contacting the substrate with an electroless nickel plating solution, the electroless nickel plating solution comprising: i) nickel ions; ii) hypophosphite ions; iii) at least one chelator; and iv) a molecule comprising divalent sulfur; wherein the electroless nickel plating solution deposits on the substrate a compressively-stressed layer of nickel comprising between 4% and 9% phosphorus.
 2. The method according to claim 1 wherein nickel ions are present in the electroless nickel plating solution at a concentration between 1 gram per liter and 5 grams per liter.
 3. The method according to claim 1 wherein nickel ions are present in the electroless nickel plating solution at a concentration between 2 gram per liter and 3 grams per liter.
 4. The method according to claim 1 wherein sodium hypophosphite is the source of hypophosphite ions and sodium hypophosphite is present in the electroless nickel plating solution at a concentration between 10 grams per liter and 25 grams per liter.
 5. The method according to claim 1 wherein sodium hypophosphite is the source of hypophosphite ions and sodium hypophosphite is present in the electroless nickel plating solution at a concentration between 15 grams per liter and 20 grams per liter.
 6. The method according to claim 1 wherein the molecule comprising divalent sulfur is selected from the group consisting of 2-aminothiazole and thiosalicylic acid.
 7. The method according to claim 1 wherein the at least one chelator is selected from the group consisting of lactic acid, malic acid, glycine, and glacial acetic acid.
 8. The method according to claim 1 wherein the electroless nickel plating solution comprises at least two chelators selected from the group consisting of lactic acid, malic acid, glycine, and glacial acetic acid.
 9. The method according to claim 1 wherein the electroless nickel plating solution further comprises bismuth ions.
 10. The method according to claim 1 wherein the electroless nickel plating solution deposits on the substrate a compressively-stressed layer of nickel comprising between 5% and 8% phosphorus.
 11. The method according to claim 1 wherein the compressively-stressed layer of nickel has a compressive stress between 200 psi and 5000 psi.
 12. The method according to claim 1 wherein nickel ions are present in the electroless nickel plating solution at a concentration between 2 gram per liter and 4 grams per liter, wherein sodium hypophosphite is the source of hypophosphite ions, wherein sodium hypophosphite is present in the electroless nickel plating solution at a concentration between 15 grams per liter and 25 grams per liter, and wherein the electroless nickel plating solution comprises at least two chelators selected from the group consisting of lactic acid, malic acid, glycine, and glacial acetic acid.
 13. The method according to claim 12 wherein the electroless nickel plating solution further comprises bismuth ions.
 14. The method according to claim 1 wherein the substrate is steel.
 15. The method according to claim 1 wherein the electroless nickel plating solution is heated to a temperature between 175° F. and 200° F.
 16. The method according to claim 1 wherein the molecule comprising divalent sulfur is thiosalicylic acid.
 17. The method according to claim 1 wherein the pH of the electroless nickel plating solution is between 4 and
 6. 18. The method according to claim 1 wherein the substrate is contacted with the electroless nickel plating solution for between 30 minutes and 2 hours. 